This invention relates to a mark signal amplifier which is used in an apparatus for electron-beam lithography and similar operations in order to detect with high accuracy the position of a mark for positioning which has been applied on the surface of a sample.
Devices for electron-beam lithography generally include a mark detecting circuit for positioning a sample (mask or wafer) and for measuring, for example, the drift and deflecting distortion of a beam deflection system. This circuit performs a very important function of making various corrections on the basis of an output signal obtained in such a manner that a mark called a "reference mark" is placed at a specified position on an X-Y stage or that a mark is applied on the sample itself, aand that the mark is scanned with an electron beam. It can be easily conjectured that the accuracy of mark detection therefore has a direct influence on the drawing accuracy itself.
Referring to FIGS. 1A, 1B and 1C, when a mark 3 (for example, a mark of evaporated Au or a mark of a step or a recess in Si) on a silicon (Si) substrate 2 is scanned by an electron beam 1, a mark detection signal A based on reflected electrons is obtained in a solid-state detector 4. This signal is subjected to a binary-coding process with a threshold signal B, and is thus turned into a pulse.
Accordingly, the mark position can be known by precisely detecting the period of time from the initiation of the scanning to the time of obtaining the pulse signal. More specifically, letting v (m/sec) denote the scanning speed of the beam and t (sec) the period of time until the binary-coded pulse is obtained, the mark position is determined as a distance v.t (m) from the starting point of the scanning. In order to find the centroid of the mark more precisely, the period of time is obtained as t=(t.sub.1 +t.sub.2 +t.sub.3 +t.sub.4)/4 where t.sub.1 and t.sub.2 denote time intervals taken for the leading edge and trailing edge, respectively, of a first pulsed signal, and t.sub.3 and t.sub.4 denote those corresponding to a pulsed signal appearing next.
In order to ensure this operation, the level of the threshold signal B must be exactly 1/2 of that of the reflected electron signal A obtained in the solid-state detector 4 when the mark is being scanned. Moreover, the signal level of the reflected electron signal A during mark scanning varies greatly depending upon the magnitude of an electron beam current, the mounting position of the sensor, the deterioration of the sensor, the contamination of the mark itself, the shape of the mark, etc. Therefore, it is extremely difficult in practice to adjust the level of the threshold signal in many cases. Further, such adjusting results in a loss in time in the drawing, requires a high degree of skill, and cannot realize a delicate adjustment of high accuracy.